Certain types of marine geophysical surveying, such as seismic or electromagnetic surveying, may include towing an energy source at a selected depth in a body of water. One or more geophysical sensor streamers may also be towed in the water at selected depths. The streamers may be long cables having geophysical sensors disposed thereon at spaced apart locations. Often, multiple streamers may be towed by a vessel, and the lateral separation between the streamers may be provided by the use of deflectors (also known as paravanes or doors). Deflectors have traditionally been large, sturdy, semi-buoyant equipment that can be reused from survey-to-survey for a number of years. Actuation of the energy source emits an energy field into the body of water. The energy field interacts with rock formations below the water bottom with changes in the energy field due to this interaction detected by geophysical sensors positioned on the sensor streamers, for instance. The detected energy may be used to infer certain properties of the subsurface rock, such as structure, mineral composition and fluid content, thereby providing information useful in the recovery of hydrocarbons. In addition to towed sensors, electromagnetic surveying may also use sensors that are substantially stationary in the body of water, which may include attachment of electromagnetic sensors on one or more cables positioned on the water bottom or attachment of the electromagnetic sensors to one or more subsurface acquisition nodes, for example.
In electromagnetic surveying, the energy source for inducing the energy fields (e.g., electromagnetic fields) include, among other devices, spaced apart electrodes or wire coils disposed along or at the end of a cable. The cable is typically towed by a vessel in the body of water. Time-varying current may be imparted across the electrodes to induce a time-varying field in the water and subsequently the subsurface rock formations. Source electrodes have traditionally been expected to deteriorate from the combination of use and exposure to sea water. In some instances, source electrodes are expected to be replaced as often as several times per year. Electromagnetic sensors may be used to measure one or more parameters (e.g., electric field components) related to a response of the subsurface rock formations to the induced electromagnetic field. Despite the data coverage that can be achieved, these types of towed systems may have drawbacks. For example, because the sensors typically only have electrodes spaced apart in the towing direction, only the inline component (e.g., inline electric field components) of the electromagnetic field may be measured. Because in-line data measurements can be less sensitive to resistor thickness at depth than cross-line data measurements depending on the depth of the water in which the survey is being conducted, use of the in-line component may result in a less well resolved image of the subsurface resistivity structure than could be obtained from the cross-line component or a combination of the two components. However, the inclusion of additional towed sources for emission of an electromagnetic field, which includes electric field components, in a cross-line direction to allow cross-line data measurements may add undesired expense and complexity to the towed system. Deploying any extra equipment typically will increase the risk and result in a more hazardous system to operate.
Accordingly, there is a need for improved methods and systems for marine electromagnetic surveys, for example, that can generate a cross-line electromagnetic field.